Included in the background of the present invention are technologies relating to heterojunction bipolar transistors (HBTs, which are electrical tilted charge devices) and light-emitting transistors, transistor lasers, and tilted charge light-emitting diodes (respectively, LETs, TLs, and TCLEDs, all of which are optical tilted charge devices). A tilted charge device gets its name from the energy diagram characteristic in the device's base region, which has, approximately, a descending ramp shape from the emitter interface to the collector (or drain, for a two terminal device) interface. This represents a tilted charge population of carriers that are in dynamic flow—“fast” carriers recombine, and “slow” carriers exit via the collector (or drain).
Regarding optical tilted charge devices and techniques, which typically employ one or more quantum size regions in the device's base region, reference can be made, for example, to U.S. Pat. Nos. 7,091,082, 7,286,583, 7,354,780, 7,535,034, 7,693,195, 7,696,536, 7,711,015, 7,813,396, 7,888,199, 7,888,625, 7,953,133, 7,998,807, 8,005,124, 8,179,937, and 8,179,939; U.S. Patent Application Publication Nos. US2005/0040432, US2005/0054172, US2008/0240173, US2009/0134939, US2010/0034228, US2010/0202483, US2010/0202484, US2010/0272140, US2010/0289427, US2011/0150487, and US2012/0068151; and to PCT International Patent Publication Nos. WO/2005/020287 and WO/2006/093883 as well as to the publications referenced in U.S. Patent Application Publication No. US2012/0068151.
An optical tilted charge device includes an active region with built-in free majority carriers of one polarity. At one input to this active region, a single species of minority carriers of opposite polarity are injected and allowed to diffuse across the active region. This active region has features that enable and enhance the conduction of majority carriers and the radiative recombination of minority carriers. On the output side of the region, minority carriers are then collected, drained, depleted or recombined by a separate and faster mechanism. Electrical contacts are coupled to this full-featured region.
In early 2004, a publication described an optical tilted charge device incorporating a quantum well in the base region of the device in order to enhance radiative recombination (see M. Feng, N. Holonyak Jr., and R. Chan, Quantum-Well-Base Heterojunction Bipolar Light-Emitting Transistor, Appl. Phys. Lett. 84, 1952, 2004). In that paper, it was demonstrated that the optical signal followed the sinusoidal electrical input signal at speeds of up to 1 GHz. More than five years later, after further work and fundamental developments (relating, among other developments, to operation methods, active area design, and epilayer structure), it was reported that high speed tilted charge devices as spontaneous emission light emitters, operated at bandwidths of 4.3 GHz (LET) and later at 7 GHz (TCLED). (See G. Walter, C. H. Wu, H. W. Then, M. Feng, and N. Holonyak Jr., Titled-Charge High Speed (7 GHz) Light Emitting Diode, Appl. Phys. Lett. 94, 231125, 2009.) Further improvements have been achieved since that time, but additional advances in efficiency and bandwidth are desirable for achieving commercially practical opto-electronic devices and techniques.
Tilted charge light-emitting devices, for example of the types disclosed in documents listed above (for example, in the form of light-emitting transistors and tilted charge light-emitting transistors and tilted charge light-emitting diodes) can produce spontaneous light emission at relatively high speed and bandwidths. For some applications, however, it would be desired to have tilted charge spontaneous light emitting devices and techniques that can operate at much higher speeds and bandwidths, and the achievement of such devices and techniques are among the objectives hereof.